1. Field of the Invention
This invention relates to a numerically controlled (NC) machine tool system capable of replacing a part of a plurality of tools stored in a tool magazine of a machine tool with a number of next to be used tools in connection with a change in the kind of workpieces to be nextly machined.
2. Description of the Prior Art
Generally, a tool magazine provided in a numerically controlled machine tool called a "machining center" has a limitation on the number of tools storeable therein. It is therefore usual that tools stored in the tool magazine are insufficient in number to machine several kinds of workpieces. To solve this problem, a known numerically controlled machine tool employs a number of exchangeable tool magazines each storing a plurality of tools prepared for respective workpieces, and the exchangeable tool magazines are selectively attached to the machine tool when the respective workpieces are machined. In the known machine tool, however, a number of the same tools are commonly used to machine several kinds of workpieces and must be stored in each of the exchangeable tool magazines. This results in inefficient use of tools. The known machine tool further requires the provision of large capacity tool magazines, the number of which corresponds to the number of kinds of workpieces to be machined in the machine tool, thus needing a huge space for installation.
In another known numerically controlled machine tool, tools are stored in a tool magazine, and only a number of tools unnecessary for subsequent machinings are successively indexed to a replacing position of the tool magazine and are successively replaced with a number of tools having been supplied to the vicinity of the tool magazine when a different kind of workpiece is next to be machined. This machine tool avoids the foregoing drawbacks. However, after the machinings of a previous kind of workpieces, the unnecessary tools are indexed to the replacing position one-by-one and are replaced with the next to be used tools, such that considerable time is consumed for tool replacement in case of many tools to be replaced. This causes the machine tool to wait for long periods of time which continue from the completion of machinings of one kind of workpiece until the initiation of machinings of other kind of workpieces.
Furthermore, in the latter type of the known numerically controlled machine tool, a tool replacing or transfer device is provided for exclusive use in the machine tool. More specifically, as disclosed in Japanese Pat. No. 53-21543 to the assignee of the present application, a frame of the transfer device is fixed on a frame of the tool magazine, and a transfer head is operably carried by the transfer device frame for replacing tools in the tool magazine with other tools having been supplied to the vicinity of the tool magazine. The machine tool system disclosed in the Japanese patent includes a number of numerically controlled machine tools, each of which has the same construction as the above-described numerically controlled machine tool, and thus requires the provision of a tool transfer device for each machine tool. This disadvantageously results not only in an increased production cost of the machine tool system, but also in complication of control of the machine tool.